Any discussion of the related art throughout the specification should in no way be considered as an admission that such prior art is widely known or forms part of common general knowledge in the field.
U.S. Pat. Nos. 5,914,709, 6,181,842 and 6,351,260 describe an optical touch screen sensor in which integrated optical waveguides are used to launch an array of light beams across a screen, then collect them at the other side of the screen and conduct them to a position-sensitive detector. In the design for the “transmit side”, an array of waveguides feeds into a row of lens elements that expand the guided light beams in the horizontal plane, then collimate them in the horizontal plane as they are launched across the screen face. Collimation in the vertical plane is achieved with an external lens (such as a cylindrical lens), however this vertical collimation is not particularly important for the purposes of this invention.
Ideally, each collimated beam in the horizontal plane should “fill” the lens with uniform power distribution, thereby producing essentially a sheet of light with arbitrarily narrow low intensity stripes corresponding to the intra-lens gaps. The waveguides are designed to be multimode, and the lens is simply designed such that the divergence angle θ of the highest order guided mode is sufficient to fill the lens (note that divergence angle increases with mode order). This “ideal” situation is illustrated in FIG. 1, which shows an array of “transmit side” lens elements 10 and associated waveguides 11. Each lens element 10 is a planar slab of dielectric material, with a curved face 12 at one end and the associated waveguide 11 at the other end. Preferably, lens element 10 and associated waveguide 11 are composed of the same material and fabricated in a unitary manner. For simplicity, waveguide 11 is preferably located symmetrically with respect to lens element 10, ie. is coincident with an axis of symmetry 13 of lens element 10. Ideally, light rays 14 from waveguide 11 enter lens element 10 at point 15 and diverge within a sector of angle θ to “fill” curved face 12, where they are refracted to form collimated output beam 16. Lens element 10 has two angled sidewalls 17 and two sidewalls 18 parallel to axis of symmetry 13, in addition to curved face 12 where refraction occurs. It will be appreciated by those skilled in the art that so long as the guided modes diverging within a lens element 10 do not encounter the sidewalls, their configuration is largely irrelevant.
In terms of waveguide structures, lens element 10 is essentially a slab waveguide in which light is confined in the out-of-plane direction, but free to diverge in-plane. In the optical path, one end of the slab waveguide is optically connected to waveguide 11, while curved surface 12 forms the other end.
The reverse process occurs on the “receive side”, which has lens elements that are mirror images of “transmit side” lens elements 10.
Referring to FIG. 2, one problem that has been encountered with the prior art design is that the collimated beams may not in fact “fill” curved faces 12 of lens elements 10, but instead form discrete beams 20 separated by considerable dark regions 21. This may occur because the actual divergence angle of the light within each lens element 10 (φ) is much smaller than the expected divergence angle (θ). In one configuration, for example, whilst θ is approximately 34°, φ is typically in the range of 10° to 16°. Without wishing to be bound by theory, it is believed that the waveguides are transmitting fewer modes than they are capable of supporting. Either the highest order modes are not being launched into the waveguides, or the highest order modes are being lost en route. Whatever the cause, the insufficient divergence causes problems in both the manufacture and performance of the touch screen. A manufacturing problem arises because when the transmitted light is in the form of discrete beams, the receive-side lens array needs to be critically aligned (in the horizontal plane) with the transmit-side lens array, so that each receive-side lens collects a discrete beam. If on the other hand the transmitted light is essentially a continuous sheet, the horizontal positioning of the receive-side lens array is non-critical. The performance problem is one of diminished spatial resolution. The detection algorithms of the touch screen sensor are capable of resolving grey scales, so that even a partial blockage of an individual beam can be detected and translated into positional information. However if the transmitted light is in the form of discrete beams, there are significant “dark” areas from which no blockage can be detected and therefore it is very difficult to interpolate with a grey scale algorithm. Additionally, the touch sensor cannot detect a touch event in the “dark” areas where there is no light transmitted. The advantages of having a continuous sheet (or “lamina”) of light rather than discrete beams are also discussed in US patent application No. 2004/0201579 A1.
One obvious solution is to increase the length of each lens element 10 such that the actual divergence angle φ is sufficient to fill each curved face 12. However in the context of optical touch screens this is often undesirable owing to the physical constraints on the width of the transmit and receive arrays (since the waveguides turn through a right angle, lens length translates directly to array width and it is necessary to fit the waveguide array in a screen bezel). For example, a reduction in divergence angle from 34° to 10° increases the lens length by a factor of approximately 3.5 so that if the lens width is 0.85 mm, its length will increase from 1.4 mm to 4.9 mm which may be impractical for an optical touch screen application given the space constraints in the bezel region of many displays.
A second problem with the prior art lens design shown in FIG. 1 is that it is a high magnification system, ie. one in which the image distance is much greater than the object distance. Such systems are well known in the art to be extremely sensitive to errors in the layout, and in particular to errors in the object distance (in this case the distance between point 15 and curved face 12) and the refractive power of the lens (determined in this case by the radius of curvature of curved face 12 and the refractive index of the material from which lens element 10 is composed). It will be appreciated by those skilled in the art that the magnification of a lens may be positive or negative, depending on whether the image it forms is erect or inverted. For the purposes of this invention, the terms “high magnification”, “low magnification” and the like should be interpreted as referring to the magnitude of the magnification.
It is therefore an object of the present invention to overcome or ameliorate at least one of the disadvantages of the prior art, or to provide a useful alternative. This invention is described from the point of view of the “transmit side” lenses, however it will be appreciated that since the “receive side” lenses are generally mirror images of the “transmit side” lenses, any inventive modifications to the design of the “transmit side” lenses will apply equally well to the “receive side” lenses.